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Encyclopedia of Laser Physics and Technology

© Dr. Rüdiger Paschotta

Last update: 2008-05-13

Definition: safety of the use of laser devices

Note: this article is meant to provide some basic information and reasonable guidelines for laser safety, rather than a precise document on official regulations. RP Photonics does not accept any legal responsibility for the correctness of details.

Hazards of Laser Light

Laser beams can be rather hazardous, particularly for the eye (sometimes also for the skin), because they can have high optical intensities even after propagation over relatively long distances. Even when the intensity at the entrance of the eye is moderate, laser radiation can be focused by the eye's lens to a small spot on the retina, where it can cause serious permanent damage within fractions of a second – even when the power level is only of the order of a few milliwatts. Possible damage mechanisms are based on thermal as well as photochemical effects. Laser damage of the eye is not always immediately noticed: it is possible e.g. to burn peripheral regions of the retina, causing blind spots which may be noticed only years later.

Eyes are particularly sensitive, but laser radiation can also cause skin injury. For infrared light, this occurs mainly via thermal effects (thermal skin burns), similar to burning the skin with other means. The penetration depth depends on the wavelength, and for such reasons a laser beam at 1.5 μm wavelength causes more pain on the skin than a 1-μm beam. While such burning should in most cases not have serious consequences, ultraviolet light can in addition induce photochemical reactions. These can lead to changes in the pigmentation, to erythema (sunburn), photokeratitis (corneal flash burns, a painful condition of the cornea) and (most importantly) to skin cancer. In the eye, UV radiation can cause photokeratitis and cataracts in the eye's lens. (For these reason, the XeCl excimer laser has acquired the nickname "cataract machine".)

How much light an eye can tolerate depends on many circumstances: not only the intensity, but also particularly the wavelength and the duration of irradiation (e.g. the pulse duration). There are detailed sets of rules for calculating safe exposure limits (maximum permissible exposure, MPE) for a given situation. Such rules are occasionally revised according to new scientific findings.

Not Only Light is Dangerous

Note that laser safety issues do not only arise from the direct effects of laser radiation, but also e.g. from

• high electric voltages in laser power supplies (e.g. for discharge lamps)
• the use of hazardous chemicals
• potentially exploding or imploding glass tubes (e.g. arc lamps)
• the risk of causing a fire
• fumes, dust, hot droplets of molten material (e.g. in laser material processing)
• secondary radiation (e.g. ultraviolet light or even X rays) when high intensity beams heat certain targets to high temperatures.

In fact, probably most victims of accidents with lasers have been hurt by such hazards (particularly by electric shocks) rather than by laser radiation.

Particularly Hazardous Situations

The following list of important safety issues can never be complete, but is meant to improve the awareness for the multitude of possible hazards:

• High-voltage power supplies can be dangerous, if workers can get into contact with the inner parts or with a defective high-voltage cable.
• Some lasers require the handling of hazardous chemicals, e.g. carcinogenic dye solutions in dye lasers. Some of these solutions can penetrate the skin, and thus need to be handled with special care.
• Near-infrared laser beams are much more hazardous than visible light with the same power level, because their radiation is focused to the retina just in the same way as visible light, whereas the blinking reflex of the human eye (normally closing the eye's lid fairly quickly when the intensity is too high) is not active. Also, no warning is possible e.g. through weak stray light: nothing can be seen until serious damage occurs.
• Ultraviolet lasers (UV lasers) endanger not only eyes, but also the skin (see above).
• Pulsed laser sources, e.g. Q-switched lasers or regenerative amplifiers, generated pulses with a peak power many orders of magnitude higher than the average output power even of a high-power laser. A single pulse from a hand-held miniature laser can totally destroy an eye.
• High-power lasers (e.g. with powers in the kilowatt region) can not only damage the eye but also the skin within short exposure times, and can easily start a fire e.g. when the beam hits materials like wood or plastics; toxic fumes may also be generated.
• In open laser setups, parasitic specular reflections (caused either by parts of the setup or by movable metallic tools, watchbands, rings, etc., but also by the residual reflectivity of anti-reflection coatings) may allow hazardous beams leaving the setup, which might hit someone's eye.
• Optical fibers, e.g. transporting high optical powers between different rooms, may release dangerous radiation when being damaged. They therefore need to be specially protected and marked.

Often less dangerous are e.g. the following cases:

• setups with low-power visible beams, where the blinking reflex of the human eye may provide sufficient protection against occasional exposure of an eye with moderate power levels
• sources operating in certain eye-safe spectral regions (e.g. the mid-infrared region with wavelength longer than ∼1.4 μm) where the light is absorbed in the eye's lens and can thus not reach the (more sensitive) retina
• fully closed laser setups with an interlock, which automatically switches off the radiation source as soon as the case is opened

Safety Classes

To give some guidance on adequate handling and required precautions, laser devices are assigned to different safety classes, with class 1 being the least dangerous one (containing e.g. lasers with microwatt power levels) and class 4 the most hazardous one. Note that the assignment to a laser safety class does not only depend on the laser power, beam quality and laser wavelength, but also on the accessibility of hazardous areas: even a high-power laser may be in a low safety class when there is no risk that dangerous radiation can leave a fully encapsulated housing.

Details like the large diameter or the divergence of involved laser beams are largely ignored in such simplified classification schemes. The concept is solely to classify the laser product itself according to some emission limits, rather than evaluating a particular setup containing a laser. The classification is indirectly based on some exposure limits for the eye (see above), but also takes into account a number of worst case assumptions concerning e.g. the distance of persons to the laser aperture, the exposure duration and the possible use of optical instruments. Therefore, the classification tends to overestimate certain risks very strongly, and a complete safety assessment has to consider the details of the whole setup and the way it is used.

Table 1: International laser safety classes, with somewhat simplified and approximate descriptions. For details, consult the applicable laser safety standard documents.

Note that there are different classification schemes (e.g. international and American ones), using classes like 1 to 4 but with somewhat different definitions. (The American system uses classes I, IA, II, IIIA, IIIB and IV similar to the classes 1 to 4 of the international system, but with significant deviations.) Particularly important standards are

• the IEC 60825-1 international laser safety standard of the International Electrotechnical Commission (IEC),
• and those based on the US user standard ANSI Z-136 (with various variations Z-136.X, in particular the Z-136.1, which has been revised in 2007)

The IEC standard has been fully adopted by the European standardization organization as EN 60825-1 and is published in national versions such as DIN EN 60825-1 in Germany. Note that these standards cover much more than only defining safety classes; they also determine the measures to be taken in order to safely work with laser products in such classes. There are also government regulations such as the relatively outdated 21 CFR 1040.10, which is still relevant for the US, although the IEC / EN standard is now also accepted there with some additions.

Generally, it is the duty of the manufacturer of a laser product to classify the product and to accordingly equip it with warning labels. However, the classification may change when a laser product is modified by a user, and the user is then responsible for reclassification.

The Nominal Hazard Zone

Originally, the required safety measures for a given laser setup where basically determined only by the safety class of the laser. As mentioned above, this classification does not reflect details such as beam divergence, which can be very relevant for safety issues: a strongly focused laser beam can be so divergent that within a moderate distance after the focus the intensities fall below the allowable exposure level for the eye. In such situations, one sometimes defines a "Nominal Hazard Zone" (NHZ) within which safe exposure levels may be exceed, in order to apply certain restricting measures to this zone instead of the whole room.

Technical Precautions

Examples for frequently used technical laser safety precautions are:

• the use of protective goggles (→ eye protection), strongly absorbing radiation with wavelengths near the laser wavelength
• full or partial encapsulation of laser setups, ideally with absorbing housing materials, avoiding specular reflections
• interlocks that automatically switch off lasers or block laser beams e.g. when a protective box or a door is opened
• key-operated switches for power supplies, preventing unauthorized use
• written warnings (indicating e.g. the types of lasers behind a door), warning lights (indicating that hazardous laser sources are operated) and automatic door locks, preventing people from entering dangerous areas
• beam stoppers (not only for main beams, but also for parasitic reflections), preventing dangerous beams from leaving the optical setup
• low-power visible pilot beams and the like, marking the paths of dangerous invisible laser beams

However, the first measure to be implemented is proper laser safety education for all staff. Personal instruction by a knowledgeable person is certainly very valuable, and should be supplemented with additional training materials such as clearly written notes, a laser safety video or DVD, etc.

Non-technical Measures

Technical measures alone are generally not sufficient for keeping safety hazards under control. A number of non-technical measures are therefore very important:

• The risks have to be carefully assessed before anything bad can happen. They need to be reassessed every time when important circumstances change, e.g. the used devices, applications, staff, details of the room, etc.
• On that basis, reasonable ways of dealing with these risks need to be developed. This involves the implementation of technical measures (see above) as well as establishing suitable working practices. The results need to be clearly described in written safety regulations.
• Adequate education needs to be ensured, so that all people who may be at risk are properly informed about both the risks and the proper ways to deal with them.
• All responsibilities need to be properly assigned and clearly defined.
• It is very important to establish a spirit which motivates all staff to take safety issues serious, recognize responsibilities for themselves and for their colleagues, suggest practical solutions, etc.

Laser Safety Regulations

Making laser safety regulations for some production facility is a difficult challenge, and for a research laboratory it is even harder. The reason is that there are partially conflicting goals:

• Regulations must be clear and understandable for those reading them.
• The rules should be sensible, ideally under all conceivable circumstances: the implied restrictions should be so that all risks are minimized without being in straight conflict with the actual goals of the work.
• The set of rules should be compact, so that people can be expected to read them carefully.

It is clear that various tradeoffs are inevitably involved, e.g. between compactness and suitability for many different circumstances, or between safety and productivity. Giving absolute priority to maximum safety while ignoring productivity and similar practical requirements will not even serve safety, because it increases the risk of regulations being ignored. It can be difficult task to analyze existing hazards and to identify the most practical way of dealing with them.

Common Obstacles

Unfortunately, reasonable laser safety regulations are either not in place or (more frequently) routinely ignored in many places such as research and development labs. Possible reasons (but no good excuses!) are:

• a lack of general knowledge on laser safety issues
• the lack of available information on specialized safety issues, e.g. related to potential hazards of ultrashort laser pulses from mode-locked lasers (hazard potential determined only by average power, or also by peak power and pulse duration?), or awareness of risks associated with fumes
• unexpected effects such as accidentally misaligned beams, vaporization of poisonous substances, defects or poor design of safety equipment, etc.
• wrong interpretation of labels like "low power laser": a 10-mW near-infrared laser may have a low power, but is still very dangerous to the eye!
• irrational assessment of risks and inappropriate judgments on working routines ("We have always done it like this!")
• the general human tendency to underestimate invisible risks, particularly when they occur over long times without apparent effects
• missing safety devices (e.g. insufficient numbers of laser goggles in situations with visitors)
• highly inconvenient, uncomfortable or otherwise impractical safety devices, e.g. laser goggles which can not be used over longer times
• excessive pressure to produce results quickly
• nonsensical laser safety regulations which undermine the awareness that the adherence to the rules is in the operator's own interest
• very formal and abstract sets of rules, obviously made primarily for avoiding legal problems for the bosses, rather than providing help in real life
• the negligence and the bad example of irresponsible supervisors, who sometimes even ridicule more responsible persons

Due to such factors, which are difficult (if impossible) to eliminate altogether, perfect laser safety (making accidents impossible) is probably impossible to reach. However, sensible regulations can greatly diminish the risks without affecting the productivity too severely. To find such sensible regulations can be a highly non-trivial task.

Bibliography

See also: eye protection, laser beams, laser pointers, lasers, ultraviolet lasers, Spotlight article 2006-12-16, Spotlight article 2007-07-01, Spotlight article 2007-07-06, Spotlight article 2007-12-18

Categories: lasers, methods